1. Technical Field
The present invention relates generally to ignition systems for internal combustion engines, and, more particularly, to an ignition system having a high resistivity core.
2. Description of the Related Art
There has been much investigation related to ignition systems for providing a spark to a combustion chamber of an internal combustion engine, as seen by reference to U.S. Pat. No. 5,706,792 issued to Boyer et al. entitled xe2x80x9cINTEGRATED IGNITION COIL AND SPARK PLUG.xe2x80x9d Boyer et al. disclose an ignition coil of the type having relatively slender dimensions suitable for being disposed in a spark plug access well, commonly referred to as a xe2x80x9cpencilxe2x80x9d coil. Boyer et al. disclose an apparatus having inherent capacitive and inductive characteristics adapted for attenuation of radio frequency interference (RFI). The apparatus of Boyer et al. includes a central core, primary and secondary coils, and an outer core or case formed of magnetic material, all coaxially arranged. While Boyer et al. teach configuring the capacitance characteristics of the ignition coil to control RFI, the capacitance associated with the ignition coil presents designers and engineers with challenges, particularly in a so-called multicharge system (i.e., delivery of multiple or repetitive sparks for a single combustion event).
One challenge involves controlling a phenomenon known in the art as a spark-on-make, or a pre-ignition condition, which is undesirable. The higher the capacitance of the ignition coil, the greater is the lead time required to charge the ignition coil. The increased charge time requires that coil charging be started earlier relative to top dead center (TDC), where pressures in the combustion chamber are reduced and therefore a voltage level required to break down a spark plug gap is also reduced. If left uncontrolled, the situation described above may increase the probability of an undesirable pre-ignition condition. Another challenge involves controlling large voltages that are produced during operation, due to leakage inductance and the like. In particular, when a primary driver coupled to a primary winding is shut off (i.e., when a spark is desired), a relatively large reflected or reverse EMF is established, for example, at a collector terminal of the driver (e.g., if it is an IGBT). As a result, a relatively expensive, and large clamp device (e.g., diode) must be used. Additionally, often a high voltage diode is used in the secondary winding circuit to block any possible spark current from flowing due to a make voltage. These high voltage devices increase cost and are large. Ignition coil capacitance bears on the selection of these devices as follows.
For a multicharge ignition coil, the level of energy that is required to be stored is proportional to the capacitance of the ignition coil itself. Applicants have determined for this invention it would therefore be desirable to lower the energy required so that a charge time can be reduced. Reducing the charge time would allow the ignition coil to be turned on closer to top dead center (TDC), where the pressures are greater, and a voltage level required to break down a spark plug gap is therefore greater. The increased gap breakdown levels would permit increased ignition on make voltages to be produced before undesirable early sparking can occur. The foregoing would allow an ignition coil design having an increased turns ratio (i.e., secondary winding NS to primary winding NP). Such an increased turns ratio would reduce reflected voltages, allowing a reduced voltage clamp device on the driver, which would reduce cost and size.
Still another problem with conventional pencil type ignition coils involves dielectric failure, particularly where the ignition coil is of the type where a secondary winding is wound on a secondary spool. Physical separations (i.e., small voids) between the inside of the secondary winding and an outer surface of the secondary spool allow for radial partial discharges across this gap. The discharges actually remove dielectric material. This process of removal continues to grow in a tree pattern, eventually permitting a short to occur. The short will fail the ignition coil, which reduces the effective service life of the product, and may increase warranty returns.
U.S. Pat. No. 6,135,099 to Marrs et al. disclose an ignition system with a transformer having an AC output connected to a spark plug with a ferrite core. Marrs et al., however, do not teach that the core is of high resistivity nor that the overall arrangement is configured to reduce capacitance.
There is therefore a need for an ignition system that addresses one or more of the challenges or minimizes or eliminates one or more of the problems set forth above.
One object of the present invention is to provide a solution to one or more of the problems or address one or more of the challenges set forth above.
One advantage of the present invention is that it provides a reduced capacitance compared to conventional ignition coils. Accordingly, a charge time is correspondingly reduced, thereby allowing charging of the ignition coil to begin closer to top dead center, where combustion chamber pressures are increased and the voltage level needed to break down the spark plug gap is also increased, thereby reducing the chance of a spark-on-make condition. Additionally, the increased voltage level permitted before a spark over can occur allows an increased turns ratio which, in turn, results in a lower reflected voltage being produced and impressed on the driver associated with the ignition coil. The reduced reflected voltage allows a reduced voltage rating for clamp circuitry or devices, which reduces cost and size.
Still another advantage of the invention relates to the reduced capacitance of the ignition coil per se, which results in a reduced level of stored energy. This provides greater flexibility over spark control during a combustion event, particularly for multicharging. Yet another advantage according to a preferred embodiment of the invention relates to improved efficiency. In such a preferred embodiment, the central core comprises high resistivity ferrite material, which exhibits reduced eddy current losses compared to, for example, conventional steel laminations. The reduced losses result in an improved overall system efficiency. Still yet another advantage according to such a preferred embodiment of the invention involves a reduced manufacturing cost compared to, for example, conventional steel laminations.
These and other objects and advantages are achieved by an ignition apparatus of the coil-on-plug type configured to be disposed in a spark plug access well. The ignition apparatus includes a central core, a primary winding, and a secondary winding wound on the core having an end (e.g., a high voltage end) coupled to a connector. The connector is configured for connection to a spark plug. In accordance with the invention, the central core is formed of high resistivity ferrite material, which reduces the ignition coil capacitance, as described in greater detail herein.
An ignition system, and a method of operating an ignition coil are also presented.